Related Field
The present invention relates to a class of electrical devices, called baluns, for achieving impedance transition from unbalanced coaxial transmission lines to balanced two-conductor transmission lines of (usually) higher impedance. In particular, the present invention relates to such baluns for use with antennas, even more particularly for use with dual polarized PCB antenna arrays.
Description of Related Art
An antenna often requires a balun, i.e. a transition from an unbalanced transmission line (e.g. micro strip line or a coaxial line) to a balanced line, in this case eventually a two-conductor line (also referred to as two-wire or twin-lead line. Often such a transition also involves, or even requires, an impedance transformer. Antennas often require baluns and this particular invention is intended to be used for instance in a phased array antenna, preferably implemented in printed circuit board (PCB) technology.
One simple example of an antenna device which requires a balun is an ordinary dipole wire antenna fed by a coaxial transmission line. A poor dipole antenna can be created by “peeling off” a bit of the screen and the dielectric at the end of a coaxial wire, bending the centre conductor an angle of 90 degrees and attaching an approximately equally long wire pointing in the opposite direction. Such a dipole will however induce currents on the outside of the cable screen resulting in unwanted radiation although it might be well matched at some frequency. A balun is required to remove these currents and to obtain a more “ideal” radiation pattern. However, since that type of dipole antenna is narrowband, only a narrowband balun is required. If a dipole is fed by a two-conductor line a balun is not required, but since two-conductor lines are impractical for transmission at microwave frequencies due to large radiation losses, they are not often used for this purpose. They are only used for very short distances, as in the case of the invention herein described.
M. C. Bailey. “Broad-Band Half-Wave Dipole”. IEEE Transactions on Antennas and Propagation, vol. 32, no. 4, April 1984 discloses a dipole antenna in front of a ground plane is described. The dipole is mentioned as broadband, with a 37% bandwidth defined by a Voltage Standing Wave Ratio, VSWR less than 2. The dipole is fed via a coaxial line and has a balun built into the design in a natural way. A bandwidth of 37% can be said to be broadband concerning a dipole but in the context of the present application we consider balun bandwidths of more than an octave, typically 3:1, to be broadband. The notion 3:1 implies that the highest frequency of operation is three times greater than the lowest ditto.
Other types of antennas requiring baluns are various types of spiral antennas such as the ones described in reference “Antenna Engineering handbook”, R. C. Johnsson (Ed.), Third ed., McGraw-Hill, 1993. chapter 14. Such antennas are usually broadband and require broadband baluns and impedance transformers. It is well known that broadband microwave devices as transformers, directional couplers etc. require volume. Sometimes they tend to occupy a space larger than desired.
In many antenna systems for transmission and reception requiring a balun, the balun is often a critical part regarding the electrical performance of the entire antenna system. Often an impedance transformer is also necessary in association with the balun and often the impedance transformer is integrated in or located close to the balun.
Often in the case of baluns, reference is made to an article considered as classical on the subject; N. Marchand. “Transmission line conversion Transformers”. Electronics, vol. 17, pp. 142-146, December 1944. In this article the word balun is not used but many different baluns are since then referred to as various types of “Marchand baluns” in reference to the author of the article: Nathan Marchand.
EP 2432072 A1 discloses a small wideband balun, allowing a predetermined impedance transmission between an asymmetrical connection point (preferably coaxial connector) and two symmetrical connection points, in a multilayer circuit for a phased array network antenna.
U.S. Pat. No. 8,283,991 B1 discloses an assembly circuit for coupling unbalanced circuits with balanced circuits, wherein the assembly includes two broadband baluns in a balun circuit as implemented on a PCB assembly. The balun circuit includes an impedance transformer for converting to higher impedance balanced circuits.
WO 2009/077791 A1 discloses a compact feed module, preferably for a dual polarized array antenna, wherein the feed module comprises a PCB feed structure for coupling signals between connections to transmitters or receivers and connection points for connecting to the antenna elements of the array antenna.
U.S. Pat. No. 7,692,601 B2 discloses a coaxial (unbalanced) to micro strip (balanced) transition for dipole antennas using a pair of baluns for feeding a dual polarized antenna with orthogonal polarization.